1. Field of the Invention
The present invention relates to electrical machines, such as computers, in general and, in particular, to electrical machines which can be serviced while performing its assigned tasks.
2. Prior Art
Because of increasing dependency on electrical machines in general, and computer-based machines in particular, there is a need for computer-based machines (hereafter called computers) which can be serviced while performing regular functions. A typical computer includes a motherboard and /or backplane with a plurality of adapter card slots. The adapter card slots are the receptacles for adapters which can be used to expand the capacity and functionality of the computer. Occasionally, adapters become defective and have to be replaced. To provide uninterrupted service, the computer has to be fully operational during removal or insertion of the adapters. In addition, the removal and/or insertion must not have deleterious results on the computer operations.
A technique termed "hot plugging", "hot swap", or variations thereof enables the insertion and/or removal of adapters while the computer is still active or operational. Even though hot plugging is a desirable goal, if not done properly, it can cause problems which over time adversely affect the operability of the computer system. A likely problem is pitting of the metal used on the connecting pins located on the adapter and the connector. The main cause of pitting is believed to be electrical arcing which occurs at the electrical contacts while interconnection is made or broken. Another problem is electrical noise which can adversely affect the performance of the system. The cause of this noise is believed to be the large change in current over a short period of time (di/dt) at the instance when the connection is made between power pins on the card and the socket. This problem is particularly severe when hot-plugging an adapter card with a large amount of decoupling capacitance. Finally, the large surge of current is likely to cause voltage transient onto the computer system backplane. The voltage transients can cause loss of data, incorrect program execution and, in severe situations, damage to delicate hardware components.
The prior art provides several systems for controlling "hot-plugging" of adapter cards. Even though the prior art systems work well for their intended purposes, they are plagued by problems which the present invention (described below) fixes. Usually, the "hot-plugging" systems have different circuit topologies and use different application methods. Notwithstanding, the differences, the different circuit topologies and/or different application methods, hereafter termed "solutions", can be classified as: umbilical cord solution, series resistance solution, series inductance solution, slow turn-on MOSFET solution and MOSFET/charge pump solution.
In the umbilical cord solution, a person uses a length of conductive wire or other conductive material (umbilical cord) for pre-charging the input capacitance of the adapter card. An auxiliary power source is required. The person uses the wire to interconnect the card to the auxiliary power source. Once the card is charged to a desired level, it is "hot-plugged" in the machine.
The drawback with the umbilical cord approach is that its success depends on the operator. If the operator inadvertently or otherwise fails to pre-charge the card or pre-charges it below an acceptable level, spikes (voltage and/or current) will result when the card is inserted in the machine. As discussed above, the spikes could damage the machine and/or cause data errors.
The series Resistance and Slow-Turn-On-MOSFET solutions require placing a resistor and MOSFET in series with the input voltage supply path. The problem with this solution is that the charge time required for pre-charging the card capacitance can be unacceptably long, especially for systems which incorporate the use of staggered pins and require stabilization of the logic supply before signal pins on the card and the backplane of the system are connected. Failure to stabilize the logic supply before connecting the signal pins could result in latch-up condition.
The series induction solution requires the placing of an inductance in series in the input voltage supply path. The solution is intended to limit the di/dt during card insertion to prevent the pins from arcing. However, the added inductance increases the likelihood of pin arcing during the removal of the card as the inductor attempts to maintain current flow during the separation of the power pin contacts.
Finally, the source-follower MOSFET (with controlled gate ramp) solution is the most recent and prevalent method used to charge capacitance on a card. This method configures a series-pass MOSFET as a source-follower whose gate is ramped to provide a controlled turn-on for the adapter card. Such topology, however, requires the implementation of a charge-pump DC/DC converter as the ramp voltage must exceed the adapter Vcc by several volts. The added converter and related circuitry not only increase the cost of the system, but also the complexity of the design. Secondly, the charge-pump's turn-on delay can be unacceptably long, especially for systems which incorporate the use of stagger pins and require the logic supply to stabilize before the signal pins are connected to avoid latchup. In such applications, the additional use of a data bus buffer switch is often required to prevent latchup condition.
Consequently, there is a need for an improved hot-plug system, hereinafter called "Hot-Swap Controller", which automatically controls the establishment of a desired voltage on a card so that the card can be hot-plugged into a machine without deleterious effects.